A spherical coordinate system is expressed by a radial distance and two angles of deviation. In the spherical coordinate system, any point on a straight line that passes through the North Pole (at 90 degrees latitude) and the South Pole (at −90 degrees latitude) is a singularity. Although angle intervals measured in a longitude direction are the same, lengths in the longitude direction corresponding to the respective angle intervals are different at different latitudes. In other words, although angle intervals measured in the longitude direction are the same, a longitude direction length corresponding to an angle interval at high latitude becomes smaller than a longitude direction length corresponding to an angle interval at low latitude. Accordingly, in the spherical grid formed by giving appropriate grid intervals respectively to latitudes and longitudes in the spherical coordinate system, a higher latitude region becomes dense with an increasing number of grid points. Accordingly, in the simulation where numerical computation is performed on each grid point based on the spherical coordinate system, inefficient computation is unavoidable in the high latitude region.
In order to allow a large-scale parallel-type simulation which is free from the above-mentioned issues specific to the spherical coordinate system and aims at reducing computation load, a new spherical structured grid for simulating various phenomena in geophysical fluid dynamics was presented in different papers at the 17th Fluid Mechanics Symposium, held in December 2003 (Paper F8-1 entitled “The “Yin-Yang Grid”: A Chimera Grid for the Spherical Geometry” presented by Akira Kageyama; Paper C6-2 entitled “Dynamical validation by shallow water equations on Yin-Yang grid” presented by Keiko Takahashi, Mitsuru Ohdaira, Akira Kageyama, and Kunihiko Watanabe; Paper C6-3 entitled “Development of a nonhydrostatic AGCM with the Yin-Yang mesh method” presented by Keiko Takahashi, Kenji Komine, Akira Kageyama, and Kunihiko Watanabe). This new spherical structured grid was also presented at the 7th Simulation Science Symposium held in October 2003 (as shown in the paper entitled “Solid Earth Simulation” presented by Akira Kageyama). This new spherical structured grid is commonly called as Yin-Yang grid. The Yin-yang grid is formed in such a manner that first and second component structured grids are complementarily combined with each other to form a sphere. The first component structured grid is constituted by extracting from a latitude-longitude grid on a first spherical coordinate system a range between a certain value of −45 degrees or less and a value of 45 degrees or more (excluding −90 and 90 degrees) in latitude and a range between a certain value of 45 degrees or less and a certain value of 315 degrees or more (excluding 0 and 360 degrees) in longitude. The first spherical coordinate system uses a z axis of an xyz space as a central axis. The second component structured grid is constituted by extracting from a latitude-longitude grid on a second spherical coordinate system a range between a certain value of −45 degrees or less and a certain value of 45 degrees or more (excluding −90 and 90 degrees) in latitude and a range between a certain value of 45 degrees or less and a certain value of 315 degrees or more (excluding 0 and 360 degrees) in longitude. The second spherical coordinate system uses a y axis of the xyz space as a central axis. First and second structured grid data are respectively arranged for the first and second component structured grids, which constitute the spherical structured grid.